Just remember 2 things:
1-) If you will spend sc then spend 50 sc a month because it will give daily login bonus. You can see it in game -> Home -> Other -> Stamp card. There the game will tell you you need to know.
2-) Use sc on stamina refill "only" if you are going to buy sc in the end because, like silver or above units, the sc comes rarely, as login bonus (5 days in a month=5sc) or at events.